The manufacturing process of stamping, forming, and assembling sheet metal workpieces often requires picking a single piece of sheet metal automatically from a stack of workpieces and loading the workpiece into a machine for processing. The sheet metal workpieces may be blanks picked up from a stack of blanks for loading into a first machine in a line of subsequent stamping, forming, or other operations. Alternatively, the sheet metal workpieces may already be stamped to form a stack of identical workpieces which are loaded into another machine. Though some processes manually pick the top sheet from a stack of blanks or workpieces, the present invention relates to an automated process for engaging and manipulating the blank workpieces.
Automated processes for picking a workpiece from a stack of blank workpieces may include various forms of programmable robotic manipulators for picking and moving the blank workpiece from a first workstation and placing the blank workpiece into a second workstation. Such automated processes for manipulating parts typically require an end-of-arm tool, which grips, holds, and releases the workpiece so that the manipulator can pick the top workpiece from the stack of workpieces and move it to its destination. A common and typical end-of-arm tool for such applications is a vacuum suction cup which utilizes compressed air and a vacuum pump or venturi-type vacuum-generating device to create vacuum within the suction cup. Rubber suction cups are utilized to contact and adhere to the blank workpiece due to the vacuum created in the vacuum cup. Once the manipulator has picked and moved the blank workpiece to its destination, the pressure in the vacuum cup is reversed, and the blank workpiece is “blown off” or released from the suction cups.
Occasionally, the manipulator picks a blank workpiece from the stack, and the next blank workpiece in the stack sticks to the first blank workpiece. This is referred to as a “double blank” condition. Factors that can cause the blank workpieces to stick together include oil or other substances, corrosion, dirt, and static electricity that may form on the workpieces. Mechanical interlocking of small metal burrs, especially on processed parts, may also cause a double blank condition. If stamping, forming, welding, or other processes call for a single blank workpiece, loading a double blank or multiple workpieces into such processes would be undesirable, as such a condition may cause damage to machinery and tooling as well as create damaged workpieces. Damaged machinery and equipment, as well as scrap workpieces, create inefficiencies that are undesirable in an industrial environment. The challenge of preventing a double blank workpiece from being loaded into a subsequent process is a major problem for the metal forming industry and other industries that handle substantially flat sheets of material.
Certain processes and equipment have been developed in the past to address this problem. For instance, powerful magnets have been utilized within close proximity of the edge of stacked workpieces. The magnets cause the rest of the stack to repel the top blank workpiece in the stack. However, such magnets only work with steel or other magnetic material and not aluminum, plastic, or other non-magnetic material.
Air knives have also been utilized in an attempt to resolve the above-noted problem. Air knives are nozzles that direct a high-pressure stream of air at the edge of the blank workpieces to displace the parts with air pressure by breaking the bond between the top blank and the adjacent blank underneath the top blank. However, the application of air knives in these situations is not consistent and not always successful.
Other electronic sensing means or devices have also been utilized to detect double blank workpieces. These electronic sensing devices are typically mounted on the end-of-arm tool, wherein the face of the sensor is forced flush with the surface of the blank workpiece so as to electronically detect if two blank workpieces are present. Again, these devices can be inconsistent in their results, while also requiring additional costs to the end-of-arm tool in that the electronic sensing device must be mounted within, and therefore sold as part of, the end-of-arm tool.
It would be desirable to provide an apparatus and method for detecting a double blank workpiece condition or multiple workpieces that is accurate and consistent without adding cost to the end-of-arm tool.